This proposal seeks to take advantage of a unique sample of siblings (participants in the Nonshared Environment in Adolescent Development project or NEAD) ideally suited to address critical issues on how sibling interaction provides an interpersonal context for the expression of deviant behavior in adolescence and adulthood. Although behavioral genetic studies have consistently shown that the shared environment has profound effects on the development of deviancy, there have been few attempts to isolate specific social processes underlying this association. Conversely, social process research has typically not evaluated family interaction from the perspective of genetic relatedness. By applying a comprehensive coding system to already collected videotaped sibling interactions in a genetically informative sample of adolescents, we propose to better characterize specific social reinforcement patterns in adolescence that serve to encourage deviant behavior, and also to determine how social influences may moderate the expression of genetic tendencies. Our specific focus is on sibling interaction, which appears to be a key shared environmental factor in adolescence. We will pursue these overall objectives through the execution of tour specific aims: (1) To utilize the genetically-informative design of the NEAD to determine the effect size of environmental and genetic contributions to social processes between siblings that are specific to reinforcement for deviancy in adolescence; (2) To employ multivariate biometrical models to determine if these specific social processes between siblings explain in part the latent social and genetic influences on deviancy typically inferred in behavioral genetic research; (3) To explore alternative models (both conceptual and computational) for associations between social influences on deviancy and genetic relatedness, including moderation models as well as tests for violations of the Equal Environments Assumption of the twin design; and (4) To test the specificity of the sibling effect model by extending prediction to other outcomes, particularly smoking, and depressive symptoms. The execution of these four specific aims will fill important gaps in our knowledge concerning social effects on deviancy, sibling influence on social development, and, perhaps most importantly, the interplay between genetic factors and interpersonal context in adolescent and adult social development. Such knowledge may be used to modify prevention and intervention programs that typically ignore the potent effects that siblings may have on the development and maintenance of deviant behavior in adolescence and into adulthood.